In typical proton exchange membrane (PEM) fuel cells, a proton conducting membrane is sandwiched between the anode and cathode and is often called a membrane electrode assembly (MEA). The membrane serves multiple purposes. It acts as an insulator for electron conduction, while conducting positive and negative charges. It also provides a solid support for the catalytic layers and separates the fuel from the oxidant feed, so that mixing or crossover does not take place.
The fuel may be delivered to the electrode in the gaseous form, for example molecular hydrogen, or liquid form, for instance methanol or formic acid dissolved in water. Oxygen, however, typically enters the cell in gaseous form, as a component of air or as pure oxygen. Due to the chemical nature of the membrane, this gives rise to logistical problems that lower cell performance. When both the fuel and the oxidant feeds are gaseous, the gasses need to be humidified so as not to dry out the PEM. If the PEM dries out, the cell performance drops considerably. In addition, the PEM needs to be kept at low temperatures, whereas the catalyst at the cathode and anode perform best at high temperatures. A cooling apparatus for the membrane is thus often needed.
Problems arise with liquid fuels as well. For instance, when methanol is introduced as an aqueous based fuel, the membrane is slightly permeable to it and crossover of the fuel to the cathode takes place. The crossover causes consumption of fuel at the cathode without production of electricity, and results in a mixed potential at the cathode, causing a considerable drop in potential.
Laminar flow fuel cells avoid the need for a PEM. In this type of cell, parallel laminar flow between two streams of liquid creates an interface between the streams, which replaces the PEM or salt bridge of conventional devices. When the first stream, containing an oxidizer, comes into contact with the first electrode, and the second stream, containing the fuel, comes into contact with the second electrode, a current is produced, while charge migration from the anode to the cathode occurs through the interface. This cell design minimizes crossover by maximizing consumption of the fuel before it diffuses into the oxidant stream.
However, in laminar flow fuel cells both fuel and oxidant are delivered in liquid form, and both the fuel and oxidant fluids must be proton conductive. This limits the applicability of oxygen as the oxidant, because this gas is characterized by a low solubility in water and aqueous solutions of electrolytes.